Controller which controls a variable optical attenuator to control the power level of a wavelength-multiplexed optical signal when the number of channels are varied

Abstract

An optical amplifier which amplifies a wavelength division multiplexed (WDM) optical signal having a variable number of channels associated with different wavelengths and outputs the amplified WDM optical signal. The optical amplifier includes (a) an optical attenuator which controls a level of the amplified WDM optical signal, and (b) a controller which controls the WDM optical signal to be amplified with an approximately constant gain.

The invention claimed is: 
     
       1. An optical transmission system comprising:
 a transmitting terminal transmitting a wavelength division multiplexed (WDM) optical signal having a variable number of channels associated with different wavelengths; and 
 an optical amplifier which amplifies the WDM optical signal from the transmitting terminal with a gain and outputs the amplified WDM optical signal, the optical amplifier including:
 an optical attenuator which controls a level of the amplified WDM optical signal, 
 an optical filter which makes the gain substantially even with respect to said different wavelengths, and 
 a controller which controls the gain to be approximately constant. 
 
 
     
     
       2. An optical transmission system as in  claim 1 , wherein the controller controls the gain to be approximately constant during variation of the number of channels in the WDM optical signal. 
     
     
       3. An optical transmission system as in  claim 1 , wherein the optical amplifier comprises:
 a first stage optical amplifier which amplifies the WDM optical signal; and 
 a second stage optical amplifier which amplifies the first stage amplified WDM optical signal. 
 
     
     
       4. An optical transmission system as in  claim 1 , wherein an attenuation level of the optical attenuator is changed to control the level of the amplified WDM optical signal. 
     
     
       5. An optical transmission system as in  claim 3 , wherein the optical attenuator is positioned between the first stage optical amplifier and the second stage optical amplifier. 
     
     
       6. An apparatus comprising:
 an optical amplifier which amplifies a wavelength division multiplexed (WDM) optical signal having a variable number of channels associated with different wavelengths with a gain and outputs the amplified WDM optical signal, the optical amplifier including:
 an optical attenuator which controls a level of the amplified WDM optical signal, 
 an optical filter makes the gain substantially even with respect to said different wavelengths, and 
 a controller which controls the gain to be approximately constant. 
 
 
     
     
       7. An apparatus as in  claim 6 , wherein the controller controls the gain to be approximately constant during variation of the number of channels in the WDM optical. 
     
     
       8. An apparatus as in  claim 6 , wherein an attenuation level of the optical attenuator is changed to control the level of the amplified WDM optical signal. 
     
     
       9. An optical transmission system comprising:
 a transmitting terminal transmitting a wavelength division multiplexed (WDM) optical signal having a variable number of channels associated with different wavelengths; 
 an optical amplifier which amplifies the WDM optical signal from the transmitting terminal with a gain and outputs the amplified WDM optical signal, the optical amplifier including:
 an optical attenuator which controls a level of the amplified WDM optical signal, 
 an optical filter making the gain substantial even with respect to said different wavelengths, and 
 a controller which controls the gain to be approximately constant; and 
 
 a receiving terminal receiving the amplified WDM optical signal from the optical amplifier. 
 
     
     
       10. An optical transmission system as in  claim 9 , wherein the controller controls the gain to be approximately constant during variation of the number of channels in the WDM optical signal. 
     
     
       11. An optical transmission system as in  claim 9 , wherein the optical amplifier comprises:
 a first stage optical amplifier which amplifies the WDM optical signal; and 
 a second stage optical amplifier which amplifies the first stage amplified WDM optical signal. 
 
     
     
       12. An optical transmission system as in  claim 9 , wherein an attenuation level of the optical attenuator is changed to control the level of the amplified WDM optical signal. 
     
     
       13. An optical transmission system as in  claim 11 , wherein the optical attenuator is positioned between the first stage optical amplifier and the second stage optical amplifier. 
     
     
       14. An apparatus comprising:
 an optical amplifier which amplifier a wavelength division multiplexed (WDM) optical signal having a variable number of channels associated with different wavelengths with a gain and outputs the amplified WDM optical signal, the optical amplifier including: 
 a first stage optical amplifier which amplifies the WDM optical signal and outputs the first stage amplified optical signal, 
 an optical attenuator which controls a level of the first stage amplified WDM optical signal and outputs the controlled WDM optical signal, 
 an optical filter making the gain substantially flat with respect to said different wavelengths, 
 a second stage optical amplifier which amplifies the controlled WDM optical signal and outputs the amplified, controlled WDM optical signal, and 
 a controller which controls the gain to be approximately constant. 
 
     
     
       15. An apparatus as in  claim 14 , wherein the controller controls the gain to be approximately constant during variation of the number of channels in the WDM optical signal. 
     
     
       16. An apparatus as in  claim 14 , wherein an attenuation level of the optical attenuator is changed to control the level of the amplified WDM optical signal.